With reference to FIG. 1, a typical optical fiber communication system includes a source 10 of information-carrying optical signals, and an optical fiber transmission line 15 for transporting the signals to at least one optical receiver 20. Where long-haul transmissions are contemplated, e.g., transmissions over distances on the order of one hundred kilometers or more, it is typical to include repeaters 25 for signal recovery and amplification. A fiber span 30 extends between each adjacent pair of repeaters, and typically also from the source to the first repeater. It is frequent practice for each fiber span to be terminated, within a repeater, by an optical amplifier 35, 40. Such an optical amplifier is typically a discrete, or locally pumped, amplifier, in the sense that the source of pump radiation is co-located with the gain medium. A typical such discrete amplifier 35 consists of a section of erbium-doped optical fiber, coupled to a semiconductor laser as a source of pump radiation.
It should be noted that the last discrete amplifier before receiver 20 (represented in the figure as amplifier 40) is often better characterized as a "preamplifier" than as a "repeater amplifier," because its primary function will often be to condition the arriving signal for reception, rather than to relay it onto a further span.
To improve economy by increasing the distance between adjacent repeaters, there has been growing interest in the use of distributed amplification in addition to discrete amplification. Amplification is said to be "distributed" if it takes place over an extended distance, and the resulting gain is, e.g., just enough to compensate for fiber loss over such distance, or in any event is of an order smaller than 0.1 dB per meter.
A distributed amplifier is typically remotely pumped, in the sense that the source of pump radiation is not co-located with the gain medium. For example, the gain medium for a Raman amplifier will often be the core of the optical fiber making up a span 30. (No doping with optically active species is necessary to make the core an effective gain medium for that purpose.) Pump radiation from a remotely situated source 50 is coupled into a span 30 from a fiber segment 55. Although not necessarily required, it is often convenient to house source 50 within the repeater just following the span to be pumped, as shown in FIG. 1. Raman pump sources are typically semiconductor lasers.
Raman amplification is described, for example, in U.S. patent application Ser. No. 08/659,607, filed on Jun. 6, 1996 by L. E. Eskildsen et al. under the title, "System and Method of Upgrading Transmission Capacity by Raman Amplification." Other forms of remotely pumped, distributed amplification, alternative to Raman amplification, have also been proposed. For example, low levels of erbium doping can be included within the cores of the optical fibers that make up fiber spans, and pumping provided from remote locations such as the repeaters. Such a technique is described, for example, in J. R. Simpson et al., "A Distributed Erbium Doped Fiber Amplifier," Paper PD-19, Proc. OFC 1990, pages PD19-1 to PD 19-4. Distributed erbium-doped amplifiers are also described in E. Desurvire, Erbium-Doped Fiber Amplifiers, Section 2.6, pages 121-136.
Additionally, it should be noted that remotely pumped amplification is not necessarily "distributed," but instead can be lumped amplification. Amplification of this kind can be implemented, for example, by remotely pumping a section of erbium-doped fiber having a moderate or high level of doping.
In response to increasing demand for information-handling capacity in optical fiber communication systems, various multiplexing techniques have been introduced. In the technique referred to as Wavelength-Division Multiplexing (WDM), multiple wavelength channels are combined on a single optical fiber. Typically, a respective optical source, such as a diode-pumped laser, is provided for each wavelength channel. A modulation device is provided for patterning the optical output from each such source. (Alternatively, the sources are directly driven by modulated signals.) Each wavelength channel potentially contains an optical carrier frequency that has been modulated to encode information. Modulation means any method for imposing data on the optical carrier, and includes, for example, amplitude modulation, frequency modulation, and phase-shift keying. A wavelength channel has a finite frequency width, which typically ranges from hundreds of MHz to tens of GHz.
The various wavelength channels are generally clustered about a central communication wavelength such as 1550 nm. For example, the International Telecommunications Union (ITU) has proposed a standard grid of wavelength channels spaced by 100 GHz and including the wavelength that corresponds to a frequency of 193.1 THz. (A channel spacing, in frequency, of 100 GHz is equivalent to a spacing in wavelength of about 0.8 nm.)
Certain difficulties arise in optical communications because of so-called non-linear effects, which arise through interactions between lightwaves of relatively high power and the transmission medium. These effects are undesirable because they can degrade the performance of the system. Although these effects occur generally in optical communication systems, they are most prevalent in WDM systems. Among WDM systems, these effects tend to be most prevalent in those systems in which there are at least ten wavelength channels, and the channel spacing is twenty times the data rate, or less.
One such non-linear effect is known as four-wave mixing. This effect tends to occur between neighboring channels, and it occurs especially in optical fibers having low dispersion within the signal band, i.e., within the range of signal wavelengths. Another such effect is cross-phase modulation. This effect also takes place as a result of interactions between different (but not necessarily adjacent) channels. However, cross-phase modulation is especially troublesome in fibers having relatively high dispersion within the signal band. Yet another non-linear effect is self-phase modulation. This effect tends to cause signal distortion within individual wavelength channels. Yet another non-linear effect is stimulated Brillouin scattering (SBS). SBS, which causes backscatter within individual wavelength channels, is especially troublesome in analog systems, where it can be a significant factor in limiting system performance.
A variety of techniques have been employed to reduce or avoid the degradation associated with nonlinear effects. For example, dispersions typically in the range of 1.5 to 8 ps/km/nm are introduced into the fiber to reduce the consequences of four-wave mixing.
However, there remains a need for optical fiber transmission systems that can handle high-capacity communications while reducing the deleterious consequences of non-linear effects.